A typical optically-amplified transmission system consists of fibre spans and amplifiers connecting transmitter and receiver equipment. In such a system overall performance is governed by the received optical signal to noise ratio (SNR), with the amplified spontaneous emission (ASE) noise being added by the amplifier chain. Amplifier power capabilities and transmission non-linearities limit the maximum signal power that may be used for effective transmission. Conventional transmission uses amplitude shift keying (ASK) to encode the data which results in a large percentage of the signal power being made up of a constant carrier component rather than all of the photons carrying data information. The transmission system is inefficient as it is amplifying this unnecessary continuous wave (CW) carrier power.
Coherent transmission has been proposed on numerous occasions as a means to enable greater transmission performance of optical systems, normally by way of increased receiver sensitivity. However, coherent receiver designs suffer from a number of problems. Conventional coherent receiver designs involve a local laser which is mixed in the correct polarisation state with an incoming signal. Heterodyne detection is performed when the laser wavelength is similar but offset by an amount from the transmit source. Homodyne detection, which gives a 3 dB advantage over heterodyne detection, requires the local oscillator (laser) to be exactly the same wavelength and phase as the source. Diversity schemes may be used for both phase and polarisation to simplify control loop requirements to maintain the optimum coherent mixing. However, the receiver structures proposed so far for coherent transmission are complicated and are not economical or practical for commercial products. In addition measured performance is usually inferior to more conventional non-return-to-zero (NRZ) or return-to-zero (RZ) equipment which has been optimised by virtue of its simplicity.
The present invention is particularly suited to optical data signals which use a pilot carrier at the signal wavelength to act as a reference for the receiver. Such a transmission technique has already been proposed (eg. Optical-Fiber Transmission P509, ISBN:0-672-22301-5), however no receiver structures have yet been designed or implemented. There are a number of ways in which a suitable data format can be produced. For example, it is known that generation of a binary phase shift keyed (BPSK) signal with a phase modulation amplitude of slightly less than 180° peak-to-peak will result in a small residual carrier component, which we refer to as a pilot carrier, in the signal. Further reductions in the modulation depth will result in pilot carrier amplitude increase. Modulation at 180° peak-to-peak will result in complete extinction of the carrier signal.